Method for making a fuel cell with large active surface and reduced volume

ABSTRACT

The present invention relates to a process for producing a fuel cell, comprising a step of forming a plurality of holes ( 10 ) in at least two substrates ( 9 ); each hole is in the seat of an individual fuel cell, the said holes having a particular geometry, such as a shape of a truncated cone or a truncated pyramid shape. 
     The various individual cells are then electrically connected by networks of electrical connections ( 11, 12 ) and are supplied via a reactant distribution network, the assembly formed by a substrate ( 9 ), the cells and the networks constituting a base module ( 9 ′). Finally, at least two base modules ( 9 ′) are assembled, the individual cells of each base module being positioned facing the individual cells of the adjacent base module(s).

TECHNICAL FIELD

The present invention relates to a process for producing a fuel cellwith a large active surface area and a small volume.

The invention therefore deals with the field of fuel cells, and moreparticularly fuel cells with a solid polymer membrane as electrolyte,such as PEMFCs (“Proton Exchange Membrane Fuel Cells”) and DMFCs(“Direct Methanol Fuel Cells”).

Fuel cells of the solid polymer electrolyte type are employed, inparticular, in land, air and water transport and in particular in landvehicles, which are currently the subject of numerous developmentprogrammes aimed at finding alternatives to the use of batteries inelectrical vehicles.

PRIOR ART

In general, fuel cells are composed of a stack of individual cells. Eachof these cells comprises an anode and a cathode positioned on eitherside of an electrolyte. The fuel, such as hydrogen H₂ for hydrogen fuelcells, is oxidized at the anode, thereby producing protons andelectrons. The electrons rejoin the external electric circuit, whereasthe protons are directed towards the cathode, through the electrolyte,which is generally in the form of an ion-conducting membrane. Oxidationby the oxidizing agent, such as atmospheric oxygen, takes place at thecathode, accompanied, in the case of hydrogen fuel cells, by theproduction of water resulting from the recombination of the ionsproduced by the reduction and of the protons.

The power densities obtained at one individual cell are very low and aregenerally insufficient to allow electrical equipment to function. It istherefore indispensable to assemble a large number of these individualcells in order to obtain a significant power. Assembly is generallyeffected by means of a stack of individual cells, the cells beingseparated by means of leaktight plates, known as bipolar plates.

Numerous configurations have already been proposed in the prior art inthe field of fuel cells.

Thus, medium-power fuel cells, i.e. fuel cells with a power of 10 to 50kW per cell, are generally put together by the “filter-press” combiningof bipolar plates made from graphite or stainless steel and membraneelectrode assemblies obtained by pressing two fabric electrodes and aproton-conducting membrane made from NAFION®.

Low-power fuel cells, i.e. with a power of 0.5 to 50 W per cell, knownas micro fuel cells, require the development of architectures andprocesses which are often derived from technologies used inmicroelectronics. The difficulty resides in assembling themicro-electrode with the thin film of proton-conducting material.Moreover, the micro-electrode must have a high electron conductivity, ahigh permeability to gas, in particular to hydrogen, in the case of aPEMFC architecture for hydrogen/air fuel cells, a high permeability togas and to methanol in the case of a DMFC architecture for methanol/airfuel cells, an ability to take the form of a thin film on a smallsurface area, and a good thermo-mechanical resistance. Themicro-electrode must also have a surface which is suitable for thedeposition of a catalyst in dispersed form.

In the literature, a distinction is drawn between architectures based onporous silicon on which a catalyst then a Nafion® membrane aresuccessively deposited in order to form the membrane electrode assembly.However, the performance of a device of this type is limited by the poorcohesion of the various layers, thereby creating a strong interfacialresistance, and by a very weak dispersion of the catalyst, the latterbeing finely divided, in order to obtain a strongly electron-conductingdeposit.

Various laboratories have developed technologies on nonporous silicon.Thus, a team from the Lawrence Livermore National Laboratory hasdeveloped a micro fuel cell by depositing, firstly, a metallic thin filmof nickel acting as an electron collector on a silicon substrate. Thecatalyst then the proton conductor are then deposited on the nickel. Thenickel is then perforated by chemical etching in order to bring intocontact the catalyst and the reducing agent, namely the hydrogen or themethanol depending on the intended fuel cell system. This technique hasa certain number of drawbacks linked in particular to the properties ofnickel. Specifically, nickel is sensitive to corrosion phenomena causedby the strongly acidic nature of the proton conductor. Moreover, thecatalyst is poorly dispersed at the perforated nickel layer, which has alow capacity to draw a homogeneous dispersion of the reducing agent overthe catalyst. Finally, this technology entails a low probability oftriple points being present.

Patent application WO 97/11503 [1] and American patent U.S. Pat. No.5,759,712 [2] describe a fuel cell architecture based on the use of amicroporous substrate impregnated with a proton-conducting material asthe central element of a micro fuel cell system. The various materialsrequired to form a fuel cell are therefore deposited on either side ofthis substrate using conventional vacuum deposition techniques. Thisinvention has two main drawbacks: firstly the fragility of the polymersubstrate, in particular when it is treated using aggressive vacuumdeposition techniques, and secondly the poor electrochemicalperformance, linked in particular to the lack of active surface area andalso to the fragility of the catalyst deposit formed directly on theproton exchange membranes.

All of the architectures presented have the particular feature of beingentirely planar, which means that it is therefore not possible to obtaina sufficiently large electrode surface area to supply portableelectronic devices with energy.

To this end, the prior art has proposed various nonplanar geometries.

U.S. Pat. Nos. 6,080,501 [3], 6,007,932 [4] and 6,001,500 [5] describe acylindrical architecture of a miniature fuel cell. This architecture isbased on winding a membrane electrode assembly of planar geometry as isconventionally employed around a mandrel made from metal foam. However,the performance of an assembly of this type is limited, for two mainreasons:

-   -   the membrane electrode assembly, which was initially planar, is        not suitable for a cylindrical geometry, which means that it is        virtually impossible to re-establish the anode-anode,        cathode-cathode and membrane-membrane contacts after the planar        membrane electrode assembly has been wound;    -   the current collectors are not in intimate contact with the        anode and the cathode, resulting in excessive interfacial        resistances.

Another American team has developed a similar tubular miniature fuelcell design. A membrane electrode assembly is wound into a cylinder.This cylinder is then integrated in a metallic “cylinder-carrier” devicemaking it possible to ensure collection of the electric current.However, this type of architecture is not suitable for portableelectronic equipment, in particular on account of the bulk generated bythe use of the “cylinder-carrier” system.

Patent JP 63 138667 [6] presents a process for realizing a nonplanarfuel cell structure, the said process consisting in depositing a cellfilm on the internal surface of a proportion of the holes in a substratein grid form. The possibility of assembling a plurality of thesesubstrates is also described.

However, this device, obtained by the process described, has thefollowing drawbacks:

-   -   on account of the configuration of the holes, and more        specifically on account of the fact that the holes are in the        shape of a parallelepiped of very low height, it is difficult to        deposit a regular cell film over the internal surface of the        said holes;    -   on account of the geometry and the arrangement of the holes in        this document, it is necessary to dedicate a proportion of the        holes in the substrate in the form of a grid to transporting the        fuel cell feed reactants, which entails a loss of surface area        for depositing the cell films and consequently a power loss in        the said cell caused by this configuration.

There is therefore a real need for a process for producing fuel cellswhich makes it possible to obtain fuel cells with the smallest possibletotal volume while preserving a large electrode active surface area andalso while allowing a network of electrical connections and a reactantdistribution network to be produced.

Moreover, there is a need for it to be possible, with this type of fuelcells, to develop electrical powers which are compatible with the saidfuel cells being used in the field, in particular, of land transport.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to propose a processfor producing a fuel cell, suitable for everyday equipment, which, interalia, meets the need referred to above and which does not have thedrawbacks, disadvantages, defects and limitations of the prior art, andwhich in particular makes it possible to produce a fuel cell with a muchlarger active surface area than its floor occupying area. Moreover, theobject of the present invention is to propose a process for producing afuel cell which makes it possible to obtain a high-power fuel cell whilesaving a significant space for forming a reactant connection andelectrical connection network in the said fuel cell.

Finally, the object of the present invention is to provide a fuel cellof reduced volume which still has a large active surface area.

This and further objects are achieved, according to the invention, by aprocess for producing a fuel cell, the said fuel cell comprising a setof individual cells which are electrically connected to one another,each individual cell comprising at least three layers, namely a membranelayer positioned between a first electrode layer and a second electrodelayer, the said process comprising, in succession,. the following steps:

-   -   a step of forming a plurality of holes in at least two        substrates, each hole opening out on either side on two opposite        faces of each substrate, via a first orifice section and a        second orifice section, and each hole having a lateral surface;    -   a step of forming individual cells on the lateral surface of        each of the said holes;    -   a step of forming, on at least one of the said opposite faces of        each substrate, a network of electrical connections and a        reactant distribution network, the said networks connecting the        individual cells to one another, the assembly formed by a        substrate, the individual cells and the said networks        constituting a base module;    -   a step of assembling at least two base modules in such a way        that the individual cells of each base module are positioned        facing the individual cells of the adjacent base module(s),    -   the said process being characterized in that, during the step of        forming the plurality of holes, each hole is formed in such a        manner that at least one out of the said first and/or second        orifice section has a surface area which is smaller than the        surface area of at least one cross section through the said hole        taken in a plane which is parallel to the said opposite faces,        and in that, for each hole, the first or second orifice section        has a surface area which is smaller than the surface area of the        other orifice section.

It should be noted that according to the invention the term “basemodule” refers to the assembly composed of a substrate within which areformed individual fuel cells, the said cells being electricallyconnected to one another via networks of electrical connections andbeing supplied via reactant distribution networks, the said networksbeing formed at at least one of the faces of the substrate on which theholes are formed.

It should be noted that according to the invention the lateral surfacedenotes the surface of the walls which delimit the hole.

It should be noted that according to the invention the reactantdistribution network denotes the network which will allow the electrodesto be supplied with oxidizing agent or reducing agent.

It should be noted that wherever reference is made to a section throughthe hole taken in a plane parallel to the opposite faces of thesubstrate, what is meant is any section apart from the abovementionedorifice sections.

It should be noted that the term substrate is preferably to beunderstood as meaning a substrate which is substantiallyparallelepipedal in shape.

It should be noted that the term active surface area, in the above textand in the text which follows, is to be understood as meaning thesurface area occupied by the electrodes, where the electrochemicalreactions take place in the fuel cell.

Holes according to the present invention may advantageously be holeswhich are substantially in the shape of a truncated cone or aresubstantially in the shape of a truncated pyramid.

The formation of the holes with a geometry of this type has the effectof bringing about the following advantages:

-   -   compared to holes with walls which are perpendicular to the        substrate, as in the case of the design of the abovementioned        Japanese document, the fact of forming holes with a profile        comprising walls which are substantially inclined with respect        to the vertical contributes to facilitating the deposition of        the layers required to form the individual cells;    -   compared to holes with walls which are perpendicular to the        substrate, the fact of forming holes with an orifice section        surface area which is smaller than the surface area of the other        orifice section makes it possible to save space at the faces        where the said holes are formed, in particular at the surface        which has the smallest orifice section surface areas; this space        saving can be devoted to forming the electrical connection        network and the reactant distribution network and/or to the        formation of additional holes in order thereby to augment the        active surface area of the fuel cell;    -   compared to holes with walls which are perpendicular to the        substrate, the lateral surface (or internal surface area of the        hole) can be larger, thereby increasing the active surface area,        in so far as the lateral surface serves as a base for the        formation of the individual cells.

It should be noted that wherever the above text refers, for comparisonpurposes, to holes with walls which are perpendicular to the substrate,these holes with perpendicular walls have an identical cross section tothe first orifice section or second orifice section referred to above.

Moreover, the advantageous nature of the present invention resides inthe fact of assembling two or more base modules in order to furtherincrease the active surface area of the resulting fuel cell.

Therefore, the benefit of the present invention is in this way to allowthe lateral surface area of the holes to be multiplied, with themarranged facing one another, by assembling at least two base modules.

Thus, by virtue of this process according to the invention, it ispossible to obtain fuel cells with a reduced floor occupying area yet alarge active surface area, in so far as the active surfaces of the fuelcell are located within the material which forms the substrate.

Moreover, the fact that the invention realizes systems derived fromassembling a plurality of modules and moreover from the specificgeometry of certain holes, has the major advantage of facilitatingproduction of the active layers on the walls of the said holes.

In fact, if one considers a fuel cell architecture with a single module,the slope of the walls, for example in the case of holes which areconical in volume, would be fixed by the geometry, in particular thethickness of the substrate, the orifice section surface areas of theholes. In a system of this type, it would be necessary to developgreatly sloping walls in order to gain active surface area. By contrast,the system derived from assembling a plurality of modules as envisagedby the process according to the invention may be composed of modules ofreduced height (than if a single module were to be used), andconsequently the internal walls of the holes may have a less steepprofile. Consequently, this system makes it easier to realize thedeposition of layers in order to form the individual cells.

According to the invention, the holes formed in each substrate can beformed by etching or alternatively by laser ablation.

The substrate may be composed, according to the invention, of a materialselected from the group consisting of silicon, such as porous silicon,graphite, ceramics and polymers.

By way of example, the ceramics may be titanium oxide or alumina and thepolymers may be Teflon®, Peek® or polysulfones.

It is preferable for each hole formed in each substrate to have a firstorifice section and a second orifice section with surface areas whichare smaller than the lateral surface of the said hole, which has theadvantage of allowing a large area of the faces of the substrate to bededicated to forming the networks of electrical connections and thereactant distribution networks.

Individual cells are formed within the holes formed in each substrate,according to the invention, by successive deposition, on the lateralsurface of each of the said holes, of at least three layers, in order toform the first electrode layer, the membrane layer and the secondelectrode layer.

This production phase may also comprise the deposition of currentcollectors at each electrode layer.

According to the invention, the assembling of two base modules, whenthis assembling places two faces without networks (namely electricalconnection network and reactant distribution network) facing oneanother, may comprise the following successive steps:

-   -   a step of applying a bonding layer to at least one of the said        faces without the said network(s); and    -   a step of joining the base modules at the said faces.

According to the invention, the assembling of at least two base modules,placing faces of which at least one is provided with a network ofelectrical connections and/or with a reactant distribution networkfacing one another, may comprise the following successive steps:

-   -   a step of masking the face(s) provided with the said network(s)        by means of an impervious and insulating layer;    -   a step of planarizing the face(s) provided with the said        network(s);    -   a step of applying a bonding layer to at least one of the faces        which are to be assembled;    -   a step of joining the said faces to be assembled of the said        base modules.

The bonding layer is preferably of identical composition to the membranelayer.

This has the advantage in particular of allowing the application, in asingle step, of the membranes to the walls of the holes and the bondinglayer at the surface.

According to a variant embodiment of the invention, the bonding layermay also be an adhesive that is different from the membrane layer and isselected from a group consisting of epoxides, polyimides, silicones,acrylic polymers.

According to another variant of the invention, the bonding layer is madefrom a material selected from silicon oxide and silicon nitride.

Once the bonding layer has been applied, the joining of two base modulesmay be performed, according to the invention, by clamping.

According to another embodiment of the invention, the joining of twobase modules may be performed by adhesive bonding.

Finally, the joining may be effected by molecular adhesion.

It is preferable for the masking step, the planarization step, theadhesive bonding step and the step of applying the bonding layer to beeffected simultaneously by application of a single layer.

According to one particularly advantageous embodiment of the invention,the single layer is a layer of identical composition to the membranelayer.

According to a variant, the single layer is a bonding layer made from amaterial selected from silicon oxide and silicon nitride.

Another object of the present invention is to propose a fuel cellobtainable by the process described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theappended drawings, in which:

FIG. 1 shows a sectional view through a hole with a geometrycorresponding to the present invention, on the lateral surface of whichan individual fuel cell has been produced using the process of theinvention.

FIG. 2 shows an isometric projection, illustrating an assembly of twobase modules (the said assembly of two base modules being referred toaccording to the terminology of the invention as “cavity level”).

FIG. 3 shows a sectional view representing an assembly which resultsfrom two cavity levels being joined side by side, the said assemblybeing obtained using a process according to the invention.

FIG. 4 represents various ways of assembling 4 base modules.

DETAILED DESCRIPTION OF THE INVENTION

The process for producing a fuel cell in accordance with the inventioncomprises, in succession, a step of forming a plurality of holes in atleast two substrates, followed by a step of forming individual cellswithin each of the holes, a step of forming, on at least one of thefaces of each substrate, a network of cathode connections, a network ofanode connections and a reactant distribution network, at the end ofwhich the assembly obtained is a base module, a step of assembling atleast two base modules, the said holes being formed in such a mannerthat at least one out of the said first and/or second orifice section ofeach hole has a surface area which is smaller than the surface area ofat least one cross section through the said hole taken in a plane whichis parallel to the said opposite faces, and in that, for each hole, thefirst or second orifice section has a surface area which is smaller thanthe surface area of the other orifice section.

The step of forming the plurality of holes at each substrate may beeffected using any known process, for example by means of an etch, suchas plasma etching or wet etching. Once the holes have been formed,individual fuel cells are applied to the lateral surface of each ofthese holes, for example by successive deposition of a first electrodelayer, a membrane layer and a second electrode layer, and if appropriateof current collectors at each of the electrode layers, on the lateralsurface of each of the said holes. According to the invention, thedeposition of the electrode layers can be effected using any knownprocess which makes it possible to obtain deposits in the form of thinfilms. This deposition may be effected, for example, by physical vapourdeposition (PVD), chemical vapour deposition (CVD), spin coating or bydipping of a base layer, for example of platinized carbon.

According to the invention, the deposition of the membrane layer may beeffected, for example, via a liquid route. The material whichconstitutes the membrane may be selected, for example, from a groupconsisting of the polyimides, the polyethersulfones, the polystyrenesand derivatives thereof, the polyether ketones and derivatives thereof,the polybenzoxazoles, the polybenzimidaoles and derivatives thereof, thepolyarylenes, such as the paraphenylenes and polyparaxylylenes.

The individual cells formed in this way are intended to be electricallyconnected in order to combine the individual electric powers of each ofthem. Moreover, these cells must be supplied with reactants. To do this,the process comprises a step of forming a network of electricalconnections and a reactant distribution network on at least one of thefaces of the substrate.

The photolithography techniques, by means of photosensitive resin orphotosensitive dry film may be used to realize these electricalconnection steps. Etching techniques may also be envisaged, inparticular heavy ion bombardment etching.

The reactant distribution network may be formed by etching channels intoat least one of the faces of the substrate, the said channels beingresponsible for routing the reactants, and it being possible for thesaid routing to be optimized by application of a diffusion layer.

FIG. 1 shows a hole 1 with a geometry in accordance with the presentinvention, on the walls of which hole are disposed layers whichconstitute an individual cell, during a step which forms part of theprocess of the invention.

According to this particular embodiment, the hole 1 is in the shape of atruncated pyramid, more specifically with a square base, and opens outon either side of opposite faces 9 a, 9 b of a substrate 9 via a firstorifice section denoted by 1 a and a second orifice section 1 b, thesurface area of the first orifice section being, in this particularcase, smaller than all other cross sections through the hole taken in aplane which is parallel to the abovementioned opposite faces, and thesaid hole having a lateral surface 1 c. This hole has a sloping internalprofile, which contributes to facilitating the step of forming theindividual cells, compared to a hole whose walls were to beperpendicular to the opposite faces of the said substrate.

The following are disposed in succession on the lateral surface 1 c ofthis hole:

-   -   an anode current collector 2, the said current collector being        connected at the surface to a network of anode connections in        the form of tracks 3;    -   a first electrode layer 4, which according to this embodiment        performs the function of an anode;    -   a membrane layer 5;    -   a second electrode layer 6, which performs the role of cathode;    -   a cathode current collector 7, connected at the surface to a        network of cathode connections in the form of tracks 8.

According to the terminology of the invention, the substrate, providedwith holes within which individual cells are formed, constitutes a basemodule, the said module being intended to be assembled with at least onefurther module so as to form at least one cavity level.

It should be noted that the term “cavity level”, where used in thedescription of the invention, refers to the assembly which results fromtwo base modules as defined above being assembled.

FIG. 2 makes it possible to understand, according to one specificembodiment of the invention, the way in which two base modules, denotedby 9′, are assembled.

Thus, this figure represents two substrates 9 which are substantiallyidentical and parallelepipedal in shape, provided with three rows ofholes 10 which are in the shape of a truncated pyramid with a squarebase. Each hole 10 constitutes an individual cell as described above inFIG. 1, with the various cells being electrically connected in series bya network of electrical connections 11, 12 (anodic and cathodic,respectively) in the form of tracks, such that the active surface areasof each individual cell are combined cumulatively. It will be obviousthat according to a variant of the invention the electrical connectionbetween the various cells may be in parallel. To facilitateillustration, the reactant distribution network is not shown in thisfigure.

Prior to assembly, according to this specific embodiment of theinvention, the faces without networks of electrical connections andreactant distribution networks of two base modules are covered with abonding layer 13 which is impervious to the reactants. This bondinglayer may, for example, be the membrane layer, used in particular forits sealing properties with respect to the reactants, but may also be alayer which has adhesive properties, the said layer being formed, forexample, by a material selected from a group consisting of epoxides,polyimides, silicones, acrylic polymers.

It should be noted that the assembling of two base modules must beperformed in such a manner that the holes of one base module arepositioned so as to face the holes of the adjacent base module(s), sothat the active surface area of a hole of one module is combinedcumulatively with the active surface area of the hole of the adjacentmodule(s). To achieve this result, the base modules intended to beassembled are, for example, positioned with the aid of a double-surfacepositioning machine with, at each module, a positioning cross system.

Once positioning has been effected, assembly is completed by a step ofjoining the two base modules, it being possible for the said step to beperformed using various techniques.

Thus, it is possible to envisage joining by clamping, in particular whenthe bonding layer applied to at least one of the faces of the modulewhich do not have networks does not have sufficient adhesive propertiesto ensure cohesive bonding of the two base modules.

The joining may also be effected by adhesive bonding. Among adhesivebonding techniques that may be envisaged, mention may be made of bondingby molecular adhesion, bonding by application of an adhesive or bywelding of polymer materials of the same type after treatment in thevicinity of the glass transition temperature. For example, if thebonding layer is of the same composition as the membrane layer, the saidmembrane being made from polymer material, the joining can be obtainedby heat treatment of the layer at a temperature greater than or equal tothe glass transition temperature of the polymer.

The assembly which results from this step of joining two base modulesconstitutes, according to the terminology of the invention, a cavitylevel.

To obtain fuel cells with an even higher ratio between the activesurface area and the floor occupying area of the said fuel cell, it ispossible, according to the invention, to envisage assembling more thantwo base modules, for example by assembling at least two cavity levelsor at least one cavity level with at least one base module.

To do this, it is advantageous for assembling of this nature, ifnecessary, to include a step of masking the networks of electricalconnections and reactant distribution networks on the faces intended tobe bonded by means of an impervious and insulating layer, a step ofplanarizing the face(s) provided with the said networks followed by astep of applying a bonding layer to at least one of the faces intendedto be bonded, and finally by a step of joining the faces in question.

These same steps can also be applied in instances where it is necessaryto assemble two base modules of which at least one of the faces to bebonded is provided with a network of anodic and/or cathodic connectionsand/or a reactant distribution network.

As its name would indicate, the masking step consists in masking thenetworks of electrical connections and the reactant distributionnetworks, in order to avoid the problems of short circuits during thebonding of the two faces and the problems of leakage of reactants.

This step is, for example, performed by the application of an imperviousand insulating layer.

The planarization step consists in making the outer faces provided withnetworks which are intended for bonding planar, for example by theapplication of a planarizing layer or by a mechanical process, such aspolishing. This planarization step is necessary in order to avoid anyproblem of surface discontinuity during assembly of the modules.

One particularly advantageous embodiment of the invention consists inperforming the masking step, the planarization step and the step ofapplying a bonding layer by application of a single layer which is, forexample, either a layer of identical composition to the membrane,consisting, for example, of Nafion®, or a layer of inorganic material,such as a material chosen from silicon oxide, silicon nitride, oralternatively a multilayer consisting of these various materials.

FIG. 3 illustrates a sectional view through a fuel cell resulting fromthe bonding of two cavity levels, obtained according to a particularembodiment of the invention.

The holes 14 formed at these various base modules are in the shape of atruncated cone, which corresponds to a hole geometry in accordance withthe present invention.

The superpositioning of layers, namely a first electrode layer 15, amembrane layer 16 and a second electrode layer 17, will be noted on thelateral surface 14 a of each hole 14.

A layer 18, corresponding to an impervious bonding layer, is responsiblefor ensuring leaktightness between two adjacent base modules 19 which,by virtue of being assembled, constitute a cavity level 20. In thisconfiguration, obtained according to a particular embodiment of theinvention, the layer 18 is of identical composition to the membranelayer 16. It should be noted that according to this particularembodiment, the assembling of two base modules 19 in order to produce acavity level 20 consists in assembling two faces without networks.

A single layer 21 which is simultaneously responsible for adhesion,leaktightness, insulation and planarization, is responsible forassembling two cavity levels 20. According to this configuration,obtained according to a particular embodiment of the invention, thesingle layer 21 is of identical composition to the membrane layer 16.

The cells of the two cavity levels are electrically connected to oneanother by way of networks of electrical connections 22, 23 in series.

The assembling of two base modules in order to form a cavity level, andalso of two cavity levels, may be envisaged in various ways.

Thus, FIGS. 4A, 4B and 4C illustrate various sectional views throughvarious forms of assembly of 4 base modules. According to theseparticular embodiments, each of the base modules comprises a pluralityof holes, the said holes being in the shape of a truncated cone.

According to FIG. 4A, each of the two cavity levels 25 results from theassembling of two base modules 24, in particular by placing the bases 27(represented by a solid line in the figure) of the holes 26 facing oneanother, the said levels then being assembled by placing the vertices 28(shown as solid lines in the figure) of the cavities thus formed facingone another.

According to FIG. 4B, each of the cavity levels 25 results from theassembling of two modules 24, in particular by placing the vertices 28of the holes 26 facing one another, the said levels then being assembledin particular by placing the bases 27 of the cavities thus formed facingone another.

Finally, according to FIG. 4C, each of the cavity levels 25 results fromthe assembling of two base modules 24 by placing the holes 26 with thebase 27 of one facing the vertex 28 of the other, the said levels thenbeing assembled by placing the cavities formed in this way with the base27 of one facing the vertex 28 of the other. These different assemblyvariants contribute to the creation of complex cavities forming the seatof individual cells which constitute the fuel cell, with a largeinternal surface area compared to the surface area of the orificesections of the resultant cavities. The result is a large active surfacearea compared to the visible surface area of the assembly formed in thisway.

The invention will now be described with reference to the followingillustrative, non-limiting example.

EXAMPLE

The objective is to develop an active surface area of 350 cm² for avisible surface area of 25 cm² and an energy of 10 Wh.

To do this, the substrate is a single-crystal silicon wafer with athickness of 400 micrometres and a visible surface area of 25 cm², onwhich is etched a network of holes. The holes are formed by plasmaetching and have a square cross section with a side length of 100micrometres, an opening surface area of 56%, the opening surface areacorresponding to the ratio between the hollow surface area and the totalsurface area, and a reduction factor of 80% between the entry surfacearea and the exit surface area of the holes. Consequently, the developedsurface area is seven times larger than the visible surface area. Thethin films required to form a fuel cell are deposited in succession onthe flanks of the holes, these films specifically comprising:

-   -   an anode comprising, in the context of the present example, a        current collector and a catalyst layer deposited by spraying of        an active ink;    -   a fine electrolyte membrane in the form of a thin film of        NAFION®, deposited by dipping;    -   a catalyst layer deposited on the membrane in order to activate        the reaction at the cathode, followed by a metallic deposition        intended to ensure collection of the electric current at the        cathode.

The networks of anodic and cathodic connections are formed, according tothis example, on one of the faces of the substrate, by means ofphotolithography techniques using photoresists and photosensitive dryfilms, and the reactant distribution network is formed by etching of thechannels. A base module is obtained at the end of these steps.

The assembling of two base modules is performed via NAFION® layers,bonded after heat treatment at a temperature which is greater than theglass transition temperature. A cavity level is obtained in this way.

The assembling of a plurality of cavity levels is performed by a silicalayer and is completed by a molecular bonding step.

It should be noted that the precise positioning of the modules or of thecavity levels intended to be assembled is effected with the aid of adouble-surface positioning machine.

1. A process for producing a fuel cell, said fuel cell comprising a setof individual cells which are electrically connected to one another,each one of said individual cells comprising at least three layersincluding a membrane layer positioned between a first electrode layerand a second electrode layer, said process comprising, in succession,the steps of: providing a plurality of substrates, each one of saidsubstrates having a first face, and a second face opposite said firstface; forming a plurality of holes in each of a corresponding one ofsaid substrates, such that each one of said holes opens out on saidfirst face and said second face of said corresponding substrate via afirst orifice section and a second orifice section, and each one of saidholes having a lateral surface; forming individual cells on said lateralsurface of each one of said holes; forming, on at least one of saidfirst face and said second face of each substrate, a network ofelectrical connections and a reactant distribution network, wherein saidnetworks connect said individual cells to one another, forming a basemodule from an assembly including said substrate, said individual cells,and said networks; a step of assembling at least two base modulesadjacent to each other such that said individual cells of each one ofsaid base modules are positioned facing said individual cells of theadjacent one or ones of said base modules, wherein, during the step offorming said plurality of holes, each one of said holes is formed suchthat at least one of said first and said second orifice sections has asurface area that is smaller than the surface area of at least onecross-section through said hole taken in a plane which is parallel tosaid opposite faces, and in that, for each hole, said first or secondorifice section has a surface area which is smaller than the surfacearea of the other orifice section.
 2. The process for producing a fuelcell according to claim 1, wherein each one of said holes issubstantially in the shape of a truncated cone.
 3. The process forproducing a fuel cell according to claim 1, wherein each one of saidholes is substantially in the shape of a truncated pyramid.
 4. Theprocess for producing a fuel cell according to claim 1, wherein each oneof said holes has a first orifice section and a second orifice sectionwith surface areas which are smaller than the lateral surface of saidone of said holes.
 5. The process for producing a fuel cell according toclaim 1, wherein each one of said holes is produced by etching.
 6. Theprocess for producing a fuel cell according to claims 1, wherein eachone of said holes is produced by laser ablation.
 7. The process forproducing a fuel cell according to claim 1, wherein said substrateconsists essentially of a material selected from a group consisting ofsilicon, porous silicon, graphite, ceramics, and a polymer.
 8. Theprocess for producing a fuel cell according to claim 1, wherein each oneof said individual cells is formed by successive deposition, on thelateral surface of each one of said holes, of at least three layers, inorder to form said first electrode layer, said membrane layer and saidsecond electrode layer.
 9. The process for producing a fuel cellaccording to claim 8, wherein the step of forming said individual cellsalso comprises the step of depositing current collectors at eachelectrode layer.
 10. The process for producing a fuel cell according toclaim 1, wherein said step of assembling two base modules includes thesteps of providing two base modules each having one face withoutnetworks and placing said two faces without networks facing one another,applying a bonding layer to at least one of said faces without networks,and joining said two base modules at said faces facing one another. 11.The process for producing a fuel cell according to claim 1, wherein insaid step of assembling at least two base modules includes the steps ofplacing one of said base modules having a face provided with saidnetwork of electrical connections and/or a reactant distribution networkfacing a face of another of said base modules, masking said face orfaces provided with said network(s) by means of an impervious andinsulating layer, planarizing said face or faces provided with the saidnetwork(s), applying a bonding layer to at least one of the faces facingone another, and joining said faces facing one another.
 12. The processfor producing a fuel cell according to claim 10 or 11, wherein thebonding layer includes a layer of identical composition to the membranelayer or a layer made from a material selected from the group consistingof silicon oxide and silicon nitride.
 13. The process for producing afuel cell according to claim 10 or 11, wherein said bonding layerincludes an adhesive chosen from epoxides, polyimides, silicones, andacrylic polymers.
 14. The process for producing a fuel cell according toclaim 10 or 11, wherein said joining step is effected by clamping. 15.The process for producing a fuel cell according to claim 10 or 11,wherein said joining step is effected by molecular adhesion.
 16. Theprocess for producing a fuel cell according to claim 10 or 11, whereinsaid joining step is effected by adhesive bonding.
 17. The process forproducing a fuel cell according to claim 11, wherein said masking step,said planarization step, and said step of applying the bonding layer areeffected simultaneously by application of a single layer.
 18. Theprocess for producing a fuel cell according to claim 17, wherein saidsingle layer has a composition identical to said membrane layer.
 19. Theprocess for producing a fuel cell according to claim 17, wherein saidsingle layer is made from a material selected from the group consistingof silicon oxide and silicon nitride.
 20. A fuel cell comprising: atleast two substrates each comprising a plurality of holes, each holeopening out on each side on two opposite faces of each substrate via afirst orifice section and a second orifice section, and each hole havinga lateral surface; individual cells formed on the lateral surface ofeach of said holes; on at least one of the said opposite faces of eachsubstrate, a network of electrical connections and a reactantdistribution network, said networks connecting said individual cells toone another, wherein a base module is comprised of an assembly of one ofsaid substrates including the individual cells and said networks, andwherein at least two of said base modules are assembled together suchthat said individual cells of one of said base modules are positionedfacing said individual cells of another of said base modules, wherein,for each hole, at least one of said first and second orifice section hasa surface area that is smaller than the surface area of at least onecross section through said hole taken in a plane which is parallel tosaid opposite faces, and in that, for each hole, said first or secondorifice section has a surface area that is smaller than the surface areaof the other orifice section.